The field of spectroscopy involves the measurement of the spectra of certain material being analyzed. Without going into detail here, which is well-known to those familiar with the art, it can in summary be said that an atom releases a quantum of energy that emits a particular wave length of the electromagnetic spectrum where bombarded by X-rays or at times by other regions of the electromagnetic spectrum.
The field of spectroscopy is that of high precision technology. The X-ray apparatus that is used to read or interpret the sample is finally dependent upon the quality of the presentation and preparation of the sample being analyzed because a precise geometry of distance is involved in the measurement process. Also, because of X-ray absorption factors, polyethylene or polypropylene films employed as X-ray transparent windows are fragile and very thin, ranging in thicknesses as low as between 0.00005 in. (0.0012 mm) to 0.00050 in. (0.013 mm).
A sample is positioned in a cylindrical sample holder that includes a holder body forming a cell adapted to contain the sample. A disposable sample holder is generally made of polyethylene or polypropylene and is generally intended to be discarded after a single use. A sample holder will range in size between diameters of 0.39 in. to 2.25 in. (1 mm to 57 mm) and have a height of about 1 in. (25 mm), although these dimensions will vary.
The first phase of the analysis is in the support and presentation of the sample for spectroscopy. In one type of holder, a very thin plastic film is placed taut as an X-ray transparent window across the circular end face of the body of the holder, and the skirt of the film is then secured to the outer wall of the body. The body is then inverted so that the window film face is positioned downwards. The holder cell is thereupon filled with the sample through the top open face of the holder body. A cap, which is not always necessary, can then be placed on the top open face to close the cell and form a complete holder of a body with a cap. The sample holder is then placed in a cell positioner of the particular spectroscopic apparatus being used, and the electromagnetic waves are applied upwardly to the downwardly positioned film face of the holder upon which the sample material is lying. The X-ray analysis may be conducted in an air, inert gas, or vacuum environment. The type of cap used, when a cap is necessary, varies in accordance with the environment and the type of sample being analyzed.
The precise geometry of distance encompassed by spectroscopic analysis is dependent upon the integrity of the film face that is tautly stretched across the bottom face of the cell of the holder. The sample material in the cell lies upon the film, which must retain a perfectly flat horizontal face, which in turn creates a perfectly flat surface of the sample material lying on the film. In addition the film must maintain a tight pressure fit around the base of the holder so as to create a seal that prevents any leakage of sample material, particularly a liquid or volatile sample, from the cell. Also, the thermal coefficient of the film and the sample can change considerably under the heat generated by the X-ray bombardment inducing shrinkage and/or expansion creating stress on the film.
Also, film can shrink or expand because of heat generated by X-ray bombardment. The coefficient of expansion of the film and the sample may vary, thus creating stress on the film.